1. Field of the Invention
The present invention relates to a MEMS element formed by etching away a sacrificial layer and including a movable portion and a smooth surface member, and a method for manufacturing the same.
2. Description of the Related Art
In general, elements including a movable portion which is formed by processing a metal layer and capable of displacement in a thickness direction are known as MEMS (Micro Electro-Mechanical System) elements, such as a variable-capacitance element and a relay element. In sensors including such movable portions, a metal layer is provided on a substrate to cover a sacrificial layer. Then, the sacrificial layer is removed by a wet etching method to form a movable portion separated from the substrate (refer to, for example, Japanese Unexamined Patent Application Publication No. 2001-347500).
Acceleration sensors as the MEMS elements include a fixed electrode provided on a substrate at a position facing the movable portion so that an acceleration acting on the sensor is detected by detecting a capacitance between the movable portion and the fixed electrode.
However, in the MEMS element described in Japanese Unexamined Patent Application Publication No. 2001-347500, the movable portion may be also corroded with an etchant (etching solution) by a wet etching method for removing the sacrificial layer. In this case, there is the problem of increasing the surface roughness of the movable portion, thereby making it impossible to precisely detect a capacitance between the movable portion and the fixed electrode.
In addition, the MEMS element may further include a cover provided to cover the movable portion by a compression-bonding method (pressure-bonding method). In this case, a smooth surface member is formed by smoothing a surface of a metal layer, and a bonding film (e.g., a gold thin film) formed on the smooth surface member is adhered, under pressure, to a bonding film formed on the cover, thereby bonding together the smooth surface member and the cover. However, when the surface of the smooth surface member is corroded by the wet etching method, the bonding film of the smooth surface member and the bonding film of the cover cannot be sufficiently adhered to each other, thereby causing a problem of defective bonding.